For many years the cosmetics industry has used products containing hydrolyzed proteins for conditioning of hair and skin. Various reasons exist for the use of degraded proteins in hair and skin care products, not the least of which is their apparent affinity for keratinous tissues. This affinity, called substantivity, is defined as the level of residual binding to the tissues after withdrawal of the product through rinsing. The residual protein associated with the tissue produces a variety of cosmetically desirable effects. For example, hair has improved texture, sheen, and resilience when conditioned with hydrolyzed proteins which bind to the hair.
Generally speaking, hydrolyzed proteins of average molecular weight in the range of from 500 to 2,000 exhibit the greatest substantivity to keratinous tissue. Lower molecular weight protein fragments or their amino acid components penetrate and absorb readily, but are more easily removed by washing or rinsing. High molecular weight or whole proteins may form films on the surface of the tissue, but size prevents absorption and makes the film susceptible to removal by washing or rinsing.
Parameters which are known to affect the substantivity of a given protein hydrolyzate include molecular weight, charge density, concentration in the solution applied to the hair, time of contact and pH. Further the condition of the tissue being treated is an important factor. The more porous the tissue, the greater is the expected degree of substantivity.
In comparing proteins of similar molecular weight, concentration and time of contact, the nature of the protein itself is of prime consideration. Proteins are polyamide polymers of amino acids and the individual amino acid moieties have acidic and basic side chains containing cationic, anionic or neutral groups. The acidic side chains have negative charge and the basic side chains have positive charge in the pH range of most hair and skin care products (pH 3 to pH 10).
Since the keratinous tissue itself is protein, its surface charge varies with pH. Generally, above pH 3.5 the surface of hair and skin has a net negative charge. There is increased ionization of carboxylate groups with subsequent deprotonation of ammonium groups. Since acidic side chains are in excess of basic side chains in keratin, the surface becomes increasingly negative with increasing pH.
The net charge of the protein used in hair treatment compositions is determined by its amino acid composition, i.e., the proportion of acidic and basic side chains. Minimizing the acidic residues or increasing the basicity of the protein results in increased substantivity, since the positive charges of the protein in the treatment solution are attracted to the negative charges of the keratinous surface by ionic interaction.
The ionic charge is of prime importance in increasing substantivity since the ionic "salt" bond is of greater strength than other surface attractions such as hydrogen bonds, van der Waal's forces, or the like. The strength of the ionic bond resists rinsing or washing, leading to improved substantivity.
In general, substantivity to keratinous tissue can be improved by selecting the optimum molecular size for the proteins used in the treatment solution, increasing protein concentration or time of contact, or using proteins of increased basicity, i.e., with positively charged side chains. For example, treatment proteins have been chemically modified through the introduction of quaternary ammonium groups, such as trimethyl ammonium or benzyl dimethyl ammonium groups, to the side chains. The stronger basicity of these groups results in increased attraction for the negatively charged keratinous surface.
Each of these approaches to increasing substantivity has limitations. Lower than optimum molecular size proteins may be more desirable in a given formula for their hygroscopic properties as a hair moisturizer. Higher than optimum molecular size proteins may be more desirable for their film forming, protective, or firming effects on the surface of hair and skin tissue. Such composition variations reduce substantivity.
Increasing the concentration of the protein increases product costs, as well as leading to formulation problems in terms of bacterial contamination, odor and tackiness of the product. The protein hydrolyzates suitable for use are limited to those which have no systemic biological activity. A variety of acidic, basic or enzymatic hydrolyzed natural proteins are commercially available for formulating hair and skin care products. For example, collagen or other animal proteins may be used as starting materials. Hydrolyzed amino acids may be derived from maize, gluten, silk or other proteins.
Increasing contact time between the protein in the treatment solution is undesirable for ease of use by the consumer. Contact time is generally limited to less than 30 minutes, particularly for hair care products.
It is desirable to increase the substantivity of commonly available cosmetic proteins so that they may be used at lower concentrations and may be of other than optimum size for maximum substantivity in order to achieve other desirable properties. Furthermore, it is desirable to have substantivity increase in short contact time. It is important that anything added to such compositions be safe and readily formulated into cosmetic products.